1. Field of the Invention
The present invention relates generally to pressure responsive devices and, more particularly, to such a device responsive to the differential pressure across a filter element to provide an exteriorly visible signal when the differential pressure exceeds a predetermined amount.
2. Description of the Prior Art
There have been many previously known fluid devices which are responsive to the differential pressure across a filter element to indicate the presence of a clogged or dirty filter element. Several of these previously known devices utilize a piston axially slidably disposed within a bore in the housing wherein the filter inlet pressure communicates with one axial end of the piston while the filter outlet pressure communicates with the other axial end.
Typically, the previously known pistons are rotatably constrained and cooperate with a twisted and flat, elongated member so that the member rotates as the piston travels along the bore. An indicator needle secured to one end of the twisted member provides an exteriorly visible indication of the axial position of the piston and, hence, the degree of clogging of the filter element. These previously known devices provide a continuous display of the degree of filter clogging. When filter clogging exceeds a predetermined amount, the filter element is replaced.
While these previously known differential fluid pressure indicators have proven adequate in operation, such devices are relatively expensive to manufacture and maintain. Moreover, for many fluid filtering applications, a signal visible only when the differential pressure across the filter element exceeds a predetermined amount is adequate without a continuous visual display of the differential pressure and, hence, of the degree of filter clogging.